1. Field of the Invention
The present invention relates to polycrystalline diamond compact cutters on a drill bit. More particularly, the present invention relates to measuring fracture toughness of a polycrystalline diamond compact cutter. The present invention also relates to rating polycrystalline diamond compact cutters for fracture toughness.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Diamond is the hardest material known, so cutters of diamond composition have been pursued. Drill bits were tipped with diamond for improved cutting efficiency through rock formations. Bonding diamond to metal is a challenge, so the drill bits evolved from steel to composite materials, in particular, tungsten carbide. Tungsten carbide composite readily bonds to diamond. Polycrystalline diamond compact (PDC) cutters are diamond tipped protrusions on the drill bit. The PDC cutters form the cutting surface of the drill bit with diamond, while the drill bit can be comprised of other materials. PDC cutters are commonly used in drilling operations for oil and gas.
A basic PDC cutter is comprised of a diamond table made from diamond grit with binder and a substrate of tungsten carbide and metallic binder, usually cobalt. The diamond grit is sintered under high temperature and high pressure conditions, forming a layer bonded to the tungsten carbide or other substrate. The High Temperature-High Pressure (HT-HP) press can form the layer with a Cobalt or other Group VIII element as the catalyst binder, and the properties of the layer have been modified for various thicknesses, profiles, and patterns to affect the working life of the cutters.
The PDC cutter is further processed to withstand the downhole conditions of extreme pressures and high temperatures. Excessive heat, over 750 degrees Celsius, causes thermal expansion of the diamond-binder bond in the diamond table, causing changes to the integrity of the cutter. To reduce the susceptibility to high temperatures, the cutter is subjected to a leaching process, which removes metallic binder from the diamond table using acid. Selective leaching removes the binder, usually cobalt, in different percentages through the volume of the diamond table so that the cutter is resistant to thermal expansion. However, the selective leaching affects other properties of the cutter, such as fracture toughness. Even though the cutter is more resistant to thermal expansion, the cutter may be less tough.
Once sintered and leached, it is important to verify the actual physical tolerances of the cutter. It is necessary to measure the fracture toughness of a cutter after manufacture, so that the appropriate cutter can be selected for a particular drill bit or for a particular position on the drill bit. A batch of cutters formed through the same sintering process and leaching process can be rated for particular fracture toughness by testing one representative cutter from the batch.
In the prior art, the method of testing fracture toughness is 3-point bending and/or 4-point bending, wherein a notch is formed on the diamond table and the amount of force to crack the cutter is found. FIG. 1 shows the method of 4-point bending with four transverse forces exerted against the diamond table surface with an induced crack. The amount of transverse force, such as the amount of weight, needed to fracture the diamond table, sets a fracture toughness measurement or K1C measurement. There are already industrial standards and tables for fracture toughness, such as ASTM (American Society for Testing and Materials International) standards, which have been developed based on this prior art method.
Various patents have issued and various applications have been published in the field of measuring PDC compact cutters.
U.S. Pat. No. 6,651,757, issued to Belnap, et al. on Nov. 25, 2003, discloses the known “drop hammer” and “3 point bending” method. Hardness is tested using a Vicker's indenter with a 500 gram load on a conventional micro-hardness testing apparatus. Measurements are taken of the impressions made on the PDC surface made by the indenter and the load. Another testing embodiment is an impact test. An insert is placed in a rigid fixture and a specific weight is dropped on it from predetermined heights. The “impact resistance” is determined as the maximum “drop height” that the insert can withstand prior to chipping or impact damage.
U.S. Pat. No. 8,322,217, issued to Bellin on Dec. 4, 2012, discloses an acoustic emissions testing device designed to determine the toughness of super hard materials. The device tests toughness by incorporating an acoustic sensor, in indenter coupled to a testing sample with a hard surface, and a load. When the load is exerted on the indenter, the force is transferred to the hard surface and the acoustic sensor detects acoustic events occurring within the testing sample. The acoustic emissions generated by the load upon the hard surface are recorded and stored for comparison to other testing samples to determine the relative toughness of those samples. The profile of acoustic emissions for a set of samples allows selection of a cutter with particular hardness.
U.S. Pat. No. 8,130,903, issued to Corbett, et al. on Mar. 6, 2012, discloses a non-destructive device and method for evaluating toughness. X-rays penetrate the sample and cause a target element, preferably a non-diamond material acting as a substrate, within the desired section of the sample to emit x-ray fluorescence. The profile of emitted fluorescence correlates to diamond content and toughness.
U.S. Pat. No. 8,404,019, issued to Ladi, et al. on Mar. 26, 2013, discloses another micro-hardness test for PDC surfaces within the context of determining the location and amount of leaching that has occurred in a PDC sample. A Lewis acid removes the metallic binder, and the solid metal in the solution is precipitated from the drained solution. The collected amount of metal indicates hardness and toughness of the cutter. United States Publication No. 20060042171, published for Radtke, et al. on Mar. 2, 2006, discloses a number of hardness tests to determine the impact resistance of ceramic impregnated superabrasives. These tests include the Knoop hardness test, a drop weight impact test, and the use of an Instron Instrumented Impact Test machine.
The prior art testing methods lack applicability in downhole conditions. The cutter does not experience such clean testing conditions downhole. For example, the cutter is not impacted straight on, but rather at an angle; and there are no induced cracks or notches on the cutter with the substrate.
Additionally, the cutter is known to have non-uniform residual stress from the formation process. After the high temperature-high pressure sintering process, there is a very high residual stress within the diamond table. The coefficient of thermal expansion of the tungsten carbide substrate is much higher than the coefficient of thermal expansion of the diamond. Given a non-planar interface on the substrate, the residual stress on the diamond table for bonding to an uneven amount of substrate would be non-uniform, after the sintering process. For diamond hard substances, the bond to the substrate is important in drilling operations, and it is known that the sintering process to bond is not uniform across the interface between the diamond table and the substrate. The prior art testing methods still only test at a center point and assume uniformity, when the formation of the PDC cutter for drilling operations conflicts with the assumption of uniformity.
It is an object of the present invention to provide a method for determining fracture toughness of a cutting element.
It is an object of the present invention to provide a method for determining fracture toughness of a cutting element at different positions on the surfaces of the cutting element.
It is an object of the present invention to provide a method for determining fracture toughness of a cutting element, when the cutting element is worn.
It is another object of the present invention to provide a method for rating fracture toughness of cutting elements for assembling a drill bit.
It is another object of the present invention to provide a method for compiling a profile of fracture toughness based on different positions on the cutting element. The profile provides information for the batch of cutters manufactured under the same conditions.
It is still another objection of the present invention to provide a method of selecting a cutting element according to the profile of fracture toughness.
These and other objectives and advantages of the present invention will become apparent from a reading of the attached specification.